The congestion charge for drivers of petrol-guzzling cars could rise to £25 - three times the current charge.

London Mayor Ken Livingstone has said he wants a sliding scale, with lower charges for low-emission vehicles and higher charges for "Chelsea tractors".

He said if his plans got wider backing, discounts could be in place by 2008, and higher charges by 2010.

Currently drivers pay £8 a day to drive into a central London zone, due to be extended westwards in February 2007.

'Chelsea tractors', many of which are responsible for some of the highest CO2 emissions of any cars on our roads, have to be dealt with

Ken Livingstone

The mayor said he wanted to encourage Londoners to buy low-emission cars to help cut carbon emissions and protect the environment.

"There is a growing sense of concern amongst Londoners about climate change caused by CO2 emissions, which is the biggest single problem facing humanity, and tackling this threat requires decisive action.

"'Chelsea tractors', many of which are responsible for some of the highest CO2 emissions of any cars on our roads, have to be dealt with."

He added that he anticipated most motorists would continue to be charged the current £8 a day.

Conservatives warned the price hike could hit small businesses and families.

Angie Bray, London Assembly Conservative leader, said: "Livingstone is now shifting the focus away from cutting congestion and onto raising money for the mayor's other congestion-causing transport projects.

"Reducing the charge during periods when the roads are quiet would be a much better way to cut congestion and therefore emissions."

The move follows the chancellor's plan to charge cars with the highest emissions more vehicle excise duty.